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178 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 179

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beam paths without changing the resonator’s optical length. How-
ever, multiple gain passes per round trip leads to more traversals of
the beam through the aberrated gain material before ejection from the
resonator, which exacerbates the OPD that would have been picked
up by the beam upon a single pass and which can limit the output
beam’s quality.

7.4.3 Master Oscillator Power Amplifiers
Master oscillator power amplifiers (MOPAs) provide versatile extrac-
tion configurations at the cost of some complexity (Fig. 7.9). A low-
power beam with well-controlled spatial and temporal characteristics
is formed using a master oscillator (MO). This beam is then amplified
separately in one or more stages of power amplifiers (PAs). Separa-
tion of the beam formation in the MO and its amplification in the PA
provides flexibility to independently optimize different output
parameters that would be impossible to generate simultaneously
from a single resonator. For example, fast pulses can be generated
from small, low-power Q-switched or mode-locked MOs without
concern for damage. Beam footprints can be optimally sized in the PA
to achieve good saturation without the need to consider resonator
mode effects. Due to the lack of feedback dynamics, it is straightfor-
ward to implement advanced methods for wavefront or polarization
correction in the PA.
Although the MOPA concept is simple, its implementation can be
cumbersome, due to the high gain often needed to bridge the orders-
of-magnitude difference in power from the MO to PA. High gain can
typically be obtained only by multiple amplifier stages or by multiple
passes per amplifier, leading to complex optical beam paths. Faraday
isolators are typically required to prevent feedback between the MO
and successive gain stages and can themselves severely limit extracted
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power due to thermal lensing and depolarization. Finally, many
MOPAs employ near-counterpropagating beam passes to reach full
saturation in the PA, imposing a requirement for some means of out-
coupling the high-power laser light through either spatial or polar-
ization multiplexing.

Master oscillator establishes direction, Power amplifier raises
wavelength, and pulse characteristics output to full power

Modulator

Figure 7.9 Master oscillator power amplifier (MOPA).